Complete the following sentence with a (compound) word derived from the one(s) given in bracket.
Old buses are major（ ）to air pollution in this city. (contribute)
【正确答案】：CONTRIBUTORS
【题目解析】：
本题考查的是常见动词的名词。
由are major可知，空缺处应填复数名词，contribute的具体性名词形式是contributor意为“贡献者、促成物”，故应填contributors。
句意：旧巴士是这个城市空气污染的主要促成物。